1. Field of the Invention
The present invention relates to magneto-optical recording media for use in recording information.
2. Description of the Prior Art
In the last few years magneto-optical disks have been developed energetically as rewritable optical disk memories, partly into practical use.
For the structure of the magneto-optical disks, the tri-layered structure has traditionally been proposed. This is a sandwich structure in which a magneto-optical layer is sandwiched on both sides by protection layers. In contrast, a quadri-layered structure, i.e. a first proposal for improvement of the foregoing has been suggested for improving the CN ratio. This is a quadri-layered structure in which a magneto-optical layer having a thickness such as to permit light to be partly transmitted and a reflection layer laid thereon are sandwiched on both sides by protection layers. (For example, see Japanese Patent Laid-Open Publications No. 87332/1980 and No. 12428/1982.) The first improvement proposal, however, has the following drawbacks:
(1) The sensitivity of writing is reduced due to the fact that the reflection layer having a high thermal conductivity is in direct contact with the magneto-optical layer.
(2) In recording with excessive power, thermal diffusion occurring in the reflection layer having a high thermal conductivity is transferred to the magneto-optical layer, directly causing the expansion of the written domain. The result is less writing power allowance.
To overcome these drawbacks, a second improvement has been proposed. This is defined by a structure in which between the magneto-optical layer and the reflection layer as shown in the first improvement proposal there is provided a dielectric layer such as silicon nitride having a thickness such as to contribute to the interference of light (over 40 nm). (For example, see Japanese Patent Laid-Open Publications No. 66549/1982, No. 52442/1984, No. 1637/1985, No. 7631/1985, No. 40542/1985, etc.). The second improvement proposal is so devised that the dielectric layer provided between magneto-optical layer and reflection layer has heat-insulating effect, which has made it possible to avoid the aforementioned two drawbacks.
However, the second improvement proposal involves more pronounced reduction in the reflectance than increase in the kerr rotation angle, thus causing readout signals to be lessened. This is due to the interference effect in the dielectric layer provided between the magneto-optical and reflection layers. The CN ratio is accordingly lowered, disadvantageously.
Consequently, the structure in which the magneto-optical layer and the reflection layer are directly laid one upon another has suffered from the problems of sensitivity reduction and writing power allowance, while that in which between the magneto-optical and reflection layers there is provided a dielectric layer having a thickness such as to contribute to the interference of light has confronted a problem that the CN ratio is lowered. More specifically, to eliminate the need of, or omit any temperature correcting apparatus for laser output, the writing power allowance in which the CN ratio is more than 50 dB is required to exceed .+-.25%. However, neither the first nor the second improvement proposal has satisfied the above requirement.